A system is described in which extracts from human cells can perform repair replication on DNA damaged by ultraviolet light or chemical carcinogens. Whole cell extracts from lymphoid cell lines are incubated with damaged plasmid DNA circles at 30 °C in the presence of ATP and the four deoxynucleoside triphosphates. Repair synthesis is monitored by the incorporation of α-32P-dATP into closed circular plasmid molecules. Analysis of the time course of the reaction suggests that the slowest step in repair is incision, rather than polymerization or ligation. The size of repair patches inserted into ultraviolet-irradiated DNA during a reaction was estimated by substitution of thymidine triphosphate with 5-bromodeoxyuridine triphosphate and sedimentation in alkaline cesium chloride gradients. Patches with heterogeneous sizes of less than 120 bases were observed.Key words: DNA repair, human cells, ultraviolet light, cell extracts.